Hot melting bonding method of PET, PE and PP multilayer composite packaging film

Abstract

The invention belongs to the technical field of packaging material processing, and particularly relates to a hot melting bonding method of a PET, PE and PP multi-layer composite packaging film. The method aims to solve the problems of solvent residue, poor interfacial compatibility, low bonding strength, thermal stress wrinkling and the like in the traditional compound process. The surface energy of each film layer is improved through multi-stage plasma surface pretreatment, and a functional nano composite adhesive containing near-infrared absorption nanoparticles is coated; the thickness of the bonding layer is controlled to be 2-5 microns by adopting micro gravure coating; after pre-pressing, high-strength near-infrared radiation is used for carrying out instantaneous selective heating on a bonding interface, so that a bonding layer is melted, and molecular chains are promoted to be diffused and entangled. Solvent-free compounding is achieved, the peel strength reaches 12 N / 15 mm or above, the production speed exceeds 150 m / min, the bonding strength, the appearance flatness and the production efficiency are remarkably improved, online nondestructive quality monitoring is supported, and the method is suitable for manufacturing of high-speed, environment-friendly and high-quality multilayer composite films.

Description

Technical Field

[0001] This application belongs to the field of packaging material processing technology, specifically relating to a hot melt bonding method for PET, PE, and PP multilayer composite packaging films. Background Technology

[0002] With the continuous advancement of polymer material technology, multilayer composite packaging films are increasingly widely used in food, pharmaceutical, and electronic products due to their ability to integrate the superior properties of multiple individual materials. By combining polymer film layers with different properties, a comprehensive performance combining high barrier properties, excellent mechanical strength, good heat-sealing properties, and printability can be obtained. In particular, composite structures composed of polyethylene terephthalate (PET), polyethylene (PE), and polypropylene (PP) aim to utilize the rigidity and barrier properties of PET, the flexibility and heat-sealing properties of PE, and the heat resistance and puncture resistance of PP, which places stringent requirements on interlayer bonding technology.

[0003] However, traditional bonding processes face numerous challenges when handling materials with vastly different physicochemical properties, such as PET, PE, and PP. Solvent-based adhesive lamination processes not only pose safety hazards due to residual organic solvents but also have long drying and curing cycles, limiting production efficiency. Meanwhile, direct hot-press lamination methods, due to the significant differences in melting points and coefficients of thermal expansion among the polymer layers, make precise temperature control difficult, easily leading to over-melting damage in the low-melting-point PE layer or weak adhesion in the high-melting-point PET layer, resulting in localized delamination or wrinkling of the composite film. Furthermore, poor compatibility between heterogeneous polymer interfaces leads to weak physical bonding at the adhesive interface, making it difficult for the composite film to meet the requirements of high-performance packaging in terms of peel strength and long-term durability.

[0004] Therefore, a hot melt bonding method for PET / PE / PP multilayer composite packaging films that can effectively improve the interlayer bonding strength of different materials is desired. Summary of the Invention

[0005] The purpose of this invention is to provide a hot-melt bonding method for PET, PE, and PP multilayer composite packaging films, which can effectively solve the problems mentioned in the background art. Existing technologies, when processing composites of polyethylene terephthalate, polyethylene, and polypropylene with vastly different physicochemical properties, face challenges such as solvent residue, low production efficiency, difficulty in precisely controlling the temperature field, poor compatibility at heterogeneous interfaces, and insufficient bonding strength. This results in defects such as delamination, wrinkling, and poor durability in the composite film.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows: a hot melt bonding method for PET, PE, and PP multilayer composite packaging films, comprising the following specific steps: Step (1) Multi-level surface pretreatment: The polyethylene terephthalate film, polyethylene film and polypropylene film to be composited are subjected to independent dielectric barrier discharge plasma surface modification treatment to improve the surface energy and chemical activity of each film layer. Step (2) Preparation and application of functional nanocomposite adhesive layer: Prepare a functional nanocomposite adhesive comprising a polar copolymer matrix, a nonpolar graft polymer and near-infrared absorbing nanoparticles, and uniformly coat the adhesive on the bonding surfaces of the pretreated polyethylene terephthalate film and the polypropylene film respectively by microgravure coating to form an adhesive layer with controlled thickness. Step (3) Primary lamination and pre-pressing: The polyethylene terephthalate film coated with adhesive layer, the polyethylene film and the polypropylene film coated with adhesive layer are stacked in sequence according to the order of polyethylene terephthalate layer, first adhesive layer, polyethylene layer, second adhesive layer and polypropylene layer, and fed into the pre-pressing roller group for primary degassing and positioning pressing to form a multi-layer composite structure to be bonded; Step (4) Selective interfacial radiation melting bonding: The multilayer composite structure to be bonded is transported to a high-intensity near-infrared radiation heating zone. By utilizing the transparency of the polymer film to a specific band of near-infrared light and the efficient absorption of the nanoparticles in the bonding layer to that band, the bonding interface is instantaneously and precisely heated to the melting temperature, which promotes the diffusion of molecular chains and physical entanglement between the bonding layer and the adjacent polymer film. Step (5) Forced cooling and curing: After the multilayer composite structure leaves the near-infrared radiation heating zone, it immediately enters a gradient cooling system composed of multiple temperature-controlled cooling rollers. Under continuous pressure, it undergoes rapid and uniform cooling to suppress interface crystallization defects, eliminate internal stress, and finally complete the curing of the high-strength composite film.

[0007] Preferably, in step (1), for the plasma treatment of polyethylene film and polypropylene film, argon is used as the carrier gas and oxygen with a volume fraction of 1% to 5% is mixed in as the reaction gas. The treatment power density is controlled at 2 to 5 watts per square centimeter, the treatment linear speed is 50 to 150 meters per minute, and the surface wetting tension after treatment reaches more than 48 dynes per centimeter.

[0008] Preferably, in step (1), for the plasma treatment of polyethylene terephthalate film, nitrogen is used as the carrier gas and ammonia with a volume fraction of 2% to 8% is mixed in as the reaction gas. The treatment power density is controlled at 3 to 6 watts per square centimeter and the treatment linear speed is 50 to 150 meters per minute, so as to introduce nitrogen-containing functional groups on its surface and enhance the chemical bonding with the adhesive.

[0009] Preferably, the functional nanocomposite adhesive in step (2) comprises, by mass percentage: 50% to 70% ethylene-vinyl acetate copolymer as a polar copolymer matrix, 20% to 40% maleic anhydride-grafted polypropylene as a non-polar graft polymer, and 1% to 5% lanthanum hexaboride nanoparticles as near-infrared absorbing nanoparticles, with the remainder being solvent. The average particle size of the lanthanum hexaboride nanoparticles is 30 to 80 nanometers.

[0010] Preferably, in step (2), the microgravure coating process has a coating roller with a screen count of 150 to 300 lines per inch, a coating speed synchronized with the main production line speed, and a drying temperature of 80 to 110 degrees Celsius, to ensure that the thickness of the dry film adhesive layer formed after the solvent has completely evaporated is 2 to 5 micrometers, and the thickness uniformity error is less than 5%.

[0011] Preferably, in step (3), the pre-pressing roller group consists of a pair of pressure rollers with silicone rubber with a hardness of Shore A70 to A90 on the surface. The pressing line pressure is set to 10 N to 30 N per meter, which aims to remove the air trapped between the layers and ensure that each film layer is tightly bonded before entering the radiation heating zone, so as to prevent uneven heat transfer caused by the existence of gaps.

[0012] Preferably, in step (4), the high-intensity near-infrared radiation heating zone is equipped with an array of halogen lamps as the radiation source, with the peak wavelength of its emission spectrum located between 950 nm and 1100 nm. The radiation energy density is controlled in a closed loop by adjusting the power supply, ranging from 80 watts to 200 watts per square centimeter. A non-contact multi-point infrared thermometer is installed below the heating zone to monitor the temperature of the composite film interface in real time and compare it with the set value, forming a feedback control loop to ensure that the interface temperature is accurately maintained between 160 degrees Celsius and 190 degrees Celsius, with an error of no more than 2 degrees Celsius.

[0013] Preferably, the heating time of near-infrared radiation in step (4), that is, the residence time of the composite film in the radiation zone, is precisely adjusted by controlling the speed of the production line, with a time range of 0.1 seconds to 0.5 seconds, so as to achieve instantaneous melting of the interface without causing overall thermal damage to high-melting-point substrates such as polyethylene terephthalate.

[0014] Preferably, the gradient cooling system in step (5) includes at least three pairs of hard chrome plating cooling rollers with circulating cooling media inside. The surface temperature of the first pair of cooling rollers is set to 70°C to 90°C for slow cooling to optimize the crystal structure; the surface temperature of the second pair of cooling rollers is set to 40°C to 60°C for rapid cooling; and the surface temperature of the third pair of cooling rollers is set to 20°C to 30°C to complete the final shaping. Each pair of cooling rollers is subjected to a linear pressure of 50 N to 100 N per meter.

[0015] In addition, the hot melt bonding method for PET, PE, and PP multilayer composite packaging film also includes an online quality monitoring step. This step is set after the gradient cooling system and uses an ultrasonic scanning probe or terahertz imaging technology to continuously and non-destructively test the composite film, identify and mark areas where the peel strength is lower than a preset threshold or where there are internal bubbles or delamination in real time, so as to achieve 100% quality monitoring of the production process.

[0016] Compared with the prior art, the present invention has the following beneficial effects: Extremely high interlayer adhesion strength and durability: By selectively modifying different polymer film layers with plasma and introducing a functional nanocomposite adhesive layer with both polar and nonpolar segments, a transformation from weak physical adsorption to strong chemical bonding and molecular chain entanglement at heterogeneous interfaces is achieved. The peel strength of the final composite film can stably reach more than 12 Newtons per 15 mm width, which is 3 to 4 times that of traditional hot pressing or adhesive bonding processes, and it can still maintain excellent adhesion stability under high temperature and high humidity environments.

[0017] Superior product quality and surface smoothness: The selective interfacial radiation melting technology employed in this invention precisely and instantaneously concentrates energy at the micron-level bonding interface, avoiding overheating or uneven heating of the overall film layer. This fundamentally solves problems such as overmelting damage of low-melting-point layers, poor adhesion of high-melting-point layers, and thermal stress wrinkling caused by significant differences in the melting points and thermal expansion coefficients of the various materials, ensuring excellent optical transparency and surface smoothness of the composite film.

[0018] Significantly improved production efficiency and environmental friendliness: This method is a solvent-free dry lamination process, completely eliminating food safety and environmental pollution problems caused by organic solvent residues. Based on the instantaneous heating characteristics of near-infrared radiation, the entire bonding process can be completed on a high-speed production line, with production line speeds reaching over 150 meters per minute. Compared to the curing cycle of traditional solvent-based lamination processes, which can last for tens of hours, production efficiency is increased by orders of magnitude.

[0019] Precise and controllable process: Through real-time monitoring and closed-loop feedback control of interface temperature, as well as gradient management of the cooling process, this invention achieves precise control of the thermodynamic and kinetic conditions of the bonding process. This high degree of process repeatability and stability ensures a high degree of consistency in product quality during large-scale production, reducing the scrap rate by more than 80%. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the overall technical solution architecture of a hot melt bonding method for a multilayer composite packaging film of PET, PE and PP proposed in this invention. Detailed Implementation

[0021] Example 1 Please refer to Figure 1 To make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to specific embodiments.

[0022] Currently, in the field of high-performance composite packaging materials, achieving high-strength, high-durability, and defect-free adhesion between polymeric film layers (such as polyethylene terephthalate, polyethylene, and polypropylene) with vastly different chemical and physical properties is a core technological challenge facing the industry. To address these technical problems, this invention proposes a highly efficient hot-melt bonding method based on selective interfacial radiation melting, and applies it to a hot-melt bonding method for PET, PE, and PP multilayer composite packaging films.

[0023] This embodiment provides a hot-melt bonding method for PET, PE, and PP multilayer composite packaging films. The entire process is integrated into an automated continuous production line, which consists of multiple serially connected functional units, including independent film roll unwinding and tension control units, multi-stage plasma surface treatment units, high-precision coating and drying units, multilayer alignment and pre-pressing units, high-intensity near-infrared radiation melting units, gradient cooling and shaping units, and online quality monitoring units. The central control system of the production line coordinates the operating speed, process parameters, and data communication of each unit to ensure the synchronization and stability of the entire lamination process. The method specifically includes the following steps: In the above-mentioned hot melt bonding method for PET, PE, and PP multilayer composite packaging films, step (1) is a multi-stage surface pretreatment. The goal of this step is to fundamentally solve the problem of poor wettability and weak adhesion caused by low surface energy and large polarity differences at the heterogeneous polymer interface by independently and specifically modifying the surfaces of the three polymer films with different chemical properties (polyethylene terephthalate film, i.e., PET film; polyethylene film, i.e. PE film; and polypropylene film, i.e. PP film). Specifically, this step is completed in three independent dielectric barrier discharge (DBD) plasma treatment stations, each dedicated to one type of film material.

[0024] For non-polar polyolefin materials such as polyethylene (PE) and polypropylene (PP) films, the core mechanism of surface pretreatment is the introduction of oxygen-containing polar functional groups. In a specific implementation scheme, the PE or PP film to be treated is drawn from the unwinding unit and continuously passed through a dielectric barrier discharge plasma treatment chamber at an atmospheric pressure environment, maintaining a constant conveying tension through a tension control system. The chamber contains at least one pair of parallel plate-shaped or coaxial cylindrical electrodes, at least one of which is covered with a dielectric material, such as quartz glass or alumina ceramic, to prevent arc discharge and ensure the generation of a uniform and stable large-area glow discharge. High-purity argon is used as the carrier gas, and its flow rate is precisely controlled by a mass flow controller. Simultaneously, oxygen with a volume fraction of 1% to 5% is mixed into the argon gas flow as a reactant gas. Under excitation by a high-frequency, high-voltage power supply (typically 10 kHz to 50 kHz), the mixed gas in the electrode gap is ionized, forming a low-temperature plasma. The plasma is rich in high-energy electrons, ions, metastable atoms, and reactive oxygen species (such as oxygen atoms and ozone). These reactive particles bombard the polymer film surface, increasing surface roughness through physical etching and forming microscopic mechanical interlocking points. More importantly, they break the CH and CC bonds on the polymer surface and graft oxygen atoms onto the broken chemical bonds, forming a large number of oxygen-containing polar functional groups such as carbonyl (C=O), hydroxyl (-OH), and carboxyl (-COOH). The processing power density is strictly controlled within the range of 2 to 5 watts per square centimeter. Too low a power density leads to insufficient modification, while too high a power density may cause thermal damage or over-etching of the film material. After this treatment, the surface energy of PE and PP films is significantly improved, and their surface wetting tension, as measured by a dyne pen, consistently reaches over 48 dynes per centimeter, thus creating conditions for good spreading and wetting of subsequent adhesives.

[0025] For polyethylene terephthalate (PET) films, the molecular structure contains ester groups, exhibiting a certain degree of polarity. However, to achieve stronger chemical bonding with specific functional groups in subsequent adhesives, different active groups need to be introduced. Therefore, a different reactive gas system was employed for the plasma treatment of PET films. Specifically, the PET film was passed through a separate DBD plasma treatment station at a speed of 50 to 150 meters per minute. This station used high-purity nitrogen as the carrier gas, mixed with 2% to 8% ammonia by volume as the reactive gas. During plasma discharge, ammonia molecules were decomposed into nitrogen-containing active groups, such as amino (-NH2) and imino (=NH). These nitrogen-containing functional groups were successfully grafted onto the surface of the PET film. The processing power density was controlled at 3 to 6 watts per square centimeter. The introduced amino and other functional groups have high chemical reactivity and can undergo nucleophilic addition reactions with maleic anhydride groups in subsequent functional nanocomposite adhesives to form stable amide chemical bonds. This upgrades the physical adsorption between the interfaces to stronger chemical bonding, greatly improving the bonding strength and durability between the PET layer and the adhesive layer.

[0026] In the above-mentioned hot melt bonding method for a PET, PE, PP multilayer composite packaging film, step (2) is the preparation and application of a functional nanocomposite adhesive layer. The core of this step is to design and apply a specially formulated adhesive that not only bridges the polar PET surface and the non-polar PE / PP surface, but also acts as a center for the absorption and conversion of near-infrared radiation energy.

[0027] First, the functional nanocomposite adhesive was prepared. The adhesive's components, by mass percentage, included 50% to 70% ethylene-vinyl acetate copolymer (EVA), 20% to 40% maleic anhydride-grafted polypropylene (MAH-g-PP), 1% to 5% lanthanum hexaboride (LaB6) nanoparticles, with the remainder being solvent (such as a mixture of toluene and ethyl acetate). The selection of each component and its mechanism of action are as follows: Ethylene-vinyl acetate copolymer (EVA) is a polar copolymer matrix. The vinyl acetate units in its molecular chain contain polar ester groups, which can form strong interactions such as hydrogen bonds with the nitrogen-containing functional groups on the surface of PET film after ammonia plasma treatment and the oxygen-containing functional groups on the surface of PE film. Maleic anhydride-grafted polypropylene (MAH-g-PP), as a non-polar grafted polymer, exhibits excellent molecular chain compatibility with PP and PE films, facilitating inter-segment diffusion and physical entanglement in the molten state. Simultaneously, the grafted maleic anhydride groups are highly reactive electrophiles, capable of chemically reacting with amino groups on the PET film surface to form amide bonds and undergoing esterification with hydroxyl groups on the PE / PP film surface. Therefore, the EVA and MAH-g-PP composite system constitutes an amphiphilic matrix, capable of generating excellent adhesion to both polar and non-polar film layers. Lanthanum hexaboride (LaB6) nanoparticles are the key functional component of this invention. As highly efficient near-infrared absorbing nanoparticles, they exhibit a strong plasmon resonance absorption effect in the 900 nm to 1200 nm wavelength range, efficiently converting light energy into heat energy, while showing weak absorption in the visible light region, ensuring the transparency of the composite film. The average particle size of the selected lanthanum hexaboride nanoparticles was controlled between 30 and 80 nanometers. This particle size range ensures sufficient near-infrared absorption efficiency while avoiding visible light scattering and sedimentation problems in the binder caused by excessive size. During preparation, EVA and MAH-g-PP were first dissolved in a mixed solvent. Then, surface-modified LaB6 nanoparticles (to improve dispersibility) were added. High-speed shear stirring and ultrasonic dispersion were used to ensure that the nanoparticles achieved uniform nanoscale dispersion in the polymer matrix, forming a stable suspension.

[0028] Subsequently, the prepared adhesive is applied to the film surface. This process employs microgravure coating technology, which is chosen for its high precision and high uniformity in thin-layer coating. Specifically, the bonding surfaces of the PET film and PP film, which have undergone pretreatment in step (1), are respectively passed through independent microgravure coating units. The core component of the coating unit is a precision-engraved coating roller with a surface covered with tiny pits (cells). The screen count of the coating roller is set to 150 to 300 lines per inch. The higher the screen count, the less ink is transferred in a single pass, resulting in a thinner and more uniform coating. During coating, the lower half of the coating roller is immersed in the adhesive trough. As it rotates, the cells are filled with adhesive, and a counter-rotating scraper precisely removes the excess material from the roller surface, leaving only a fixed amount of adhesive in the cells. When the film passes through the pressure zone between the coating roller and the back pressure roller at a speed synchronized with the main production line, the adhesive in the cells is precisely transferred to the film surface. The coated wet film then enters a multi-segment hot air drying oven. The drying oven temperature is set between 80 and 110 degrees Celsius, a temperature sufficient to allow the solvent to evaporate quickly without causing premature cross-linking of the adhesive or deformation of the substrate. By precisely controlling the screen count of the coating rollers, the coating speed, the adhesive viscosity, and the drying process, a dry film adhesive layer with a thickness of 2 to 5 micrometers and a thickness uniformity error of less than 5% is finally formed on the surfaces of both PET and PP films. One adhesive layer is used for bonding PET to PE, and another layer is used for bonding PE to PP.

[0029] In the above-mentioned hot melt bonding method for PET, PE, and PP multilayer composite packaging film, step (3) is primary lamination and pre-pressing. The purpose of this step is to precisely stack the independently processed and coated film layers together in a predetermined structural order and perform preliminary compaction to prepare for the subsequent melt bonding step.

[0030] Specifically, film layers from three different unwinding and processing paths converge here. Following the requirements of the final product structure—the order of polyethylene terephthalate layer, first adhesive layer, polyethylene layer, second adhesive layer, and polypropylene layer—a precise guide roller system and alignment control system (such as a CCD camera-based edge detection system) precisely stack the PET film coated with the first adhesive layer, the uncoated PE film, and the PP film coated with the second adhesive layer together. The coated side of the PET film faces the PE film, and the coated side of the PP film also faces the PE film. The stacked multilayer structure is immediately fed into a pre-pressing roller assembly. This assembly consists of a pair of pressure rollers coated with silicone rubber with a Shore A70 to A90 hardness. Silicone rubber is chosen as the roller surface material because of its good elasticity and temperature resistance, providing uniform pressure distribution without damaging the film material. The pressing line pressure is set at a low level, ranging from 10 N to 30 N per meter. The primary purpose of applying pressure in this step is not to achieve adhesion, but rather to fulfill two key preparatory functions: first, to eliminate air that may be trapped between layers during the lamination process, preventing defects such as bubbles and delamination caused by air expansion during subsequent heating; second, to ensure a tight fit between the film layers, guaranteeing the absence of micro-gaps when entering the radiant heating zone. These gaps would become poor mediums for heat conduction, leading to uneven heat transfer and affecting the uniformity of the final adhesion quality. After pre-pressing, a stable, tightly connected multilayer composite structure to be bonded is formed. This structure, under the traction of the tension system, smoothly enters the next processing unit.

[0031] In the above-mentioned hot melt bonding method for PET, PE, and PP multilayer composite packaging films, step (4) is selective interfacial radiation fusion bonding. This is the core and key innovation of the entire method, which utilizes the differences in the absorption of electromagnetic waves of specific wavelengths by materials to achieve precise and instantaneous energy transfer to the bonding interface.

[0032] The multilayer composite structure to be bonded is continuously fed into a high-intensity near-infrared (NIR) radiation heating zone. This heating zone is structurally equipped with an array of halogen lamps as a high-power radiation source. These halogen lamps are specially designed so that the spectrum emitted by their tungsten filaments at high temperatures is precisely controlled, resulting in a peak wavelength between 950 nm and 1100 nm. The fundamental reason for choosing this wavelength is that the three polymer substrates—PET, PE, and PP—have extremely high optical transmittance and extremely low absorption in this near-infrared band. Therefore, the radiant energy can penetrate the bulk film, which has a total thickness of tens to hundreds of micrometers, with almost no attenuation. However, the adhesive layer distributed at the two bonding interfaces, due to the uniform dispersion of lanthanum hexaboride nanoparticles, exhibits extremely strong absorption characteristics in this near-infrared band. Therefore, when near-infrared radiation irradiates the composite film, the energy is selectively and almost entirely absorbed within the two-micrometer-thick adhesive layer and converted into heat energy within nanoseconds to picoseconds, causing the temperature of the adhesive layer to rise rapidly to its melting temperature in a very short time.

[0033] To achieve precise control of the bonding process, a complex closed-loop feedback control system is integrated into the heating zone. Below the heating zone, on the other side of the composite film's path, a non-contact multi-point infrared thermometer (or infrared thermal imager) is installed. This thermometer monitors and collects real-time surface temperature distribution data of the composite film as it leaves the heating zone. Due to the instantaneous nature of the heating process and the thinness of the film, this surface temperature can highly correlate with the actual temperature of the internal interface. The thermometer feeds the real-time temperature data back to the central controller, which compares it with a preset target interface temperature (typically set between 160 and 190 degrees Celsius) and calculates the deviation. Subsequently, the controller dynamically adjusts the power supply to the halogen lamp array using a PID (proportional-integral-derivative) algorithm, thereby precisely controlling the radiant energy density. The radiant energy density is controlled within a range of 80 to 200 watts per square centimeter. This closed-loop control ensures that the temperature of the bonding interface is precisely maintained near the target value, with a dynamic error of no more than 2 degrees Celsius.

[0034] Furthermore, the total heating time at the interface, i.e., the residence time of the composite film in the radiation zone, is achieved by precisely controlling the operating speed of the production line. By coordinating the length of the heating zone with the speed of the production line, the residence time is controlled within an extremely short range of 0.1 to 0.5 seconds. The advantage of this instantaneous heating strategy is that it can bring the adhesive layer to a molten state, promoting sufficient interdiffusion, penetration, and physical entanglement between its polymer chains and the molecular chains at the interface of adjacent PET, PE, and PP film layers, forming a strong adhesive interface. At the same time, it avoids overall thermal damage, thermal shrinkage, or deformation to high-melting-point substrates such as PET (approximately 260 degrees Celsius) and low-melting-point substrates such as PE (approximately 110-130 degrees Celsius). This fundamentally solves the problem of balancing heating conditions due to the large differences in the melting points of materials in traditional hot-pressing composite processes.

[0035] In the above-mentioned hot melt bonding method for PET, PE, and PP multilayer composite packaging film, step (5) is forced cooling and shaping curing. After instantaneous melting and molecular chain rearrangement occur at the interface, a controlled cooling process is used to "lock" the newly formed interface structure and eliminate internal stress, ultimately giving the composite film stable physical properties and excellent appearance.

[0036] Specifically, after the multilayer composite structure leaves the near-infrared radiation heating zone, it is immediately introduced into a gradient cooling system consisting of multiple temperature-controlled cooling rollers. This system includes at least three pairs of hard chrome-plated cooling rollers internally circulated with a cooling medium (such as water or thermal oil). The reason for using gradient cooling instead of scalding at a single temperature is to precisely control the crystallization behavior of the composite film.

[0037] The surface temperature of the first pair of cooling rollers is set within a relatively high range, from 70 to 90 degrees Celsius. When the high-temperature composite film first comes into contact with these "warm" cooling rollers, its cooling rate is relatively slow. This slow cooling process provides sufficient time for the polymer molecular chains in the interfacial region to align properly, optimize the crystalline structure, and form larger, more complete crystals, thereby maximizing the mechanical strength and toughness of the interface.

[0038] Subsequently, the composite film enters the second pair of cooling rollers, where the surface temperature is set in a medium range, namely 40 to 60 degrees Celsius. At this stage, the cooling rate is significantly accelerated, mainly to rapidly cool the main part of the film layer, allowing it to quickly pass through the glass transition temperature or crystallization temperature range, thereby fixing its macroscopic dimensions and preventing warping or wrinkling caused by uneven crystallization shrinkage that may occur due to slow cooling.

[0039] Finally, the composite film passes through a third pair of cooling rollers, with its surface temperature set in the range close to room temperature, i.e., 20 to 30 degrees Celsius, to complete the final shaping and curing, ensuring that the temperature of the composite film has been sufficiently reduced and the structure is completely stable before winding.

[0040] Throughout the cooling process, each pair of cooling rollers applies a continuous and uniform linear pressure to the composite membrane, with the pressure value set at 50 to 100 Newtons per meter. This pressure ensures that the composite membrane remains in close contact with the surface of the cooling rollers during cooling and shrinkage, achieving maximum heat exchange efficiency and uniform cooling, while further compacting the interface to prevent any potential delamination.

[0041] Furthermore, the hot-melt bonding method of this invention integrates an online quality monitoring step as the final product quality control checkpoint. The execution unit for this step is located after the gradient cooling system and before the winding unit. The monitoring system employs advanced non-destructive testing technologies, such as high-frequency ultrasonic scanning probe arrays or terahertz time-domain spectroscopy imaging technology. These probes are arranged along the width of the film, performing a 100% full-width continuous scan of the high-speed composite film. Ultrasonic or terahertz waves can penetrate multilayer composite films, and when encountering defects such as poor interfacial adhesion, internal microbubbles, delamination, or impurities, the acoustic impedance, amplitude, or phase of the echo signal will undergo characteristic changes. The host of the detection system analyzes these signals in real time, and by comparing them with a pre-set qualified product signal model, it can accurately identify areas where the peel strength is below a preset threshold or where internal defects exist. Once a non-conforming area is detected, the system automatically triggers a marking device (such as an inkjet printhead or laser marker) to mark the corresponding position on the composite film or directly record the defect coordinate information. This enables 100% quality control of the production process, providing a precise basis for subsequent slitting and rejecting of defective products, thereby greatly improving the reliability and consistency of product quality.

[0042] The method of this embodiment will be illustrated below through a specific application example.

[0043] In a scenario where composite films are used to produce retortible food packaging bags, the specifications of the raw film used are as follows: 12-micron thick biaxially oriented PET film, 50-micron thick linear low-density PE film, and 20-micron thick cast PP film.

[0044] The production line speed is set to 120 meters per minute.

[0045] In step (1), for PE and PP films, the process parameters of the plasma treatment station were set as follows: argon flow rate of 50 standard liters per minute, mixed with 3% volume fraction of oxygen, and a treatment power density of 4 watts per square centimeter. For PET films, the treatment station parameters were set as follows: nitrogen flow rate of 50 standard liters per minute, mixed with 5% volume fraction of ammonia, and a treatment power density of 5 watts per square centimeter. After treatment, the surface wetting tension of all films reached 52 dynes per centimeter.

[0046] In step (2), the formulation of the functional nanocomposite adhesive is as follows: 60% by mass of EVA (VA content 28%), 35% by mass of MAH-g-PP (grafting rate 1%), 3% by mass of LaB6 nanoparticles (average particle size 50 nm), and the remainder is solvent. The microgravure coating roller is configured with 200 lines per inch, the drying temperature is set to 95 degrees Celsius, and the final dry film adhesive layer thickness is 3.5 micrometers.

[0047] In step (3), the linear pressure of the pre-pressed roller assembly is set to 20 N per meter.

[0048] In step (44), the power of the halogen lamp array in the near-infrared radiation heating zone is automatically adjusted by the closed-loop control system to maintain the interface temperature precisely stable at 180 degrees Celsius. Based on the linear velocity and the length of the heating zone, the residence time of the composite film in the radiation zone is calculated to be 0.25 seconds.

[0049] In step (5), the surface temperatures of the three pairs of cooling rollers are set to 85 degrees Celsius, 55 degrees Celsius and 25 degrees Celsius respectively, and the linear pressure applied to each pair of cooling rollers is 80 Newtons per meter.

[0050] The final PET / PE / PP multilayer composite packaging film, after testing, showed that its interlayer peel strength was stable at over 13.5 Newtons per 15 mm width, its appearance was smooth and wrinkle-free, its optical transparency was high, and it passed the 121°C, 30-minute boiling test without delamination or shrinkage. The product qualification rate reached over 99.5%.

[0051] Example 2 This embodiment provides another specific implementation of a hot-melt bonding method for PET, PE, and PP multilayer composite packaging films. The overall process flow of this embodiment is basically the same as that of Embodiment 1, but different technical solutions are used in the material system of the functional nanocomposite adhesive layer and the heat source implementation method for selective interfacial radiation melting, aiming to demonstrate the applicability and flexibility of the present invention under different technical paths. For steps not described in detail in this embodiment, their specific implementation methods are the same as or similar to those described in Embodiment 1.

[0052] In this embodiment, the preparation and application of the functional nanocomposite adhesive layer in step (2) were adjusted. The composition of the adhesive was redesigned to accommodate different supply chain or cost requirements. Specifically, its components, by mass percentage, include: 55% to 75% ethylene-methyl acrylate-maleic anhydride terpolymer, 15% to 35% chlorinated polypropylene, and 0.5% to 3% antimony-doped tin dioxide (ATO) nanoparticles, with the remainder being solvent.

[0053] In this formulation, the ethylene-methyl acrylate-maleic anhydride terpolymer integrates both polar groups (methyl acrylate groups) and reactive groups (maleic anhydride), enabling it to simultaneously establish effective physical and chemical interactions with modified surfaces of PET, PE, and PP. Chlorinated polypropylene acts as a powerful adhesion promoter, particularly for PP and PE surfaces, significantly enhancing interfacial adhesion.

[0054] The key functional substitution lies in the use of antimony-doped tin dioxide (ATO) nanoparticles as the near-infrared absorber in this embodiment. ATO nanoparticles exhibit stronger absorption peaks in longer near-infrared bands (e.g., 1400 nm to 2000 nm). Therefore, their average particle size is selected in the range of 20 nm to 60 nm to obtain optimal absorption efficiency and dispersion stability. The preparation process is similar to that of Example 1, using high-energy dispersion technology to uniformly mix the components in a solvent.

[0055] The parameters of the coating and drying process were adjusted accordingly based on the rheological properties of the new adhesive and the solvent system, but the final dry film adhesive layer thickness, uniformity and other indicators remained consistent with the requirements of Example 1.

[0056] Accordingly, this embodiment also modifies the technical implementation of selective interfacial radiation melting bonding in step (4). Since the near-infrared absorber in the bonding layer is changed to ATO nanoparticles, its optimal absorption band changes. Therefore, different radiation sources are configured in the high-intensity near-infrared radiation heating zone. In this embodiment, a high-power semiconductor laser diode array is used as the radiation source, and its emission center wavelength is precisely set at 1550 nm to accurately match the absorption peak of the ATO nanoparticles.

[0057] Compared to halogen lamps, laser diodes offer advantages such as narrower spectral width, higher energy density, more controllable beam shape, and higher electro-optical conversion efficiency. By shaping and homogenizing the beam emitted from the laser diode array using an optical system, a linear or rectangular heating region with extremely uniform energy distribution can be formed along the operating path of the composite film.

[0058] The control system also employs a closed-loop feedback mode. A non-contact infrared thermometer monitors the interface temperature and feeds the data back to the laser driver power controller. The controller uses high-speed pulse width modulation (PWM) to precisely adjust the output power of the laser diode in real time at the microsecond level, resulting in higher control accuracy of the interface temperature and keeping fluctuations within 1 degree Celsius. The radiation energy density can be adjusted between 100 watts and 300 watts per square centimeter according to production requirements. Due to the higher energy concentration of laser heating, the heating time can be further shortened, controlled within the range of 0.08 seconds to 0.4 seconds, which is more advantageous for protecting heat-sensitive substrates.

[0059] By adopting the above-mentioned alternative technical solutions, this embodiment can also achieve high-quality hot-melt bonding of PET / PE / PP multilayer composite packaging films. In a specific test, the composite film produced using the method of this embodiment achieved performance indicators such as peel strength, retort resistance, and surface smoothness that were comparable to or even surpassed those of Example 1 in some aspects (such as temperature control accuracy). This demonstrates that the core technical idea of ​​this invention (i.e., selective energy absorption through a functional nanocomposite adhesive layer and precise interfacial radiation melting) has broad implementation possibilities and excellent process robustness. The basic principles, main features, and advantages of this invention have been shown and described above. Those skilled in the art should understand that this invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of this invention. Various changes and modifications can be made to this invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the invention as claimed. The scope of protection of this invention is defined by the appended claims and their equivalents.

Claims

1. A hot-melt bonding method for a multilayer composite packaging film of PET, PE, and PP, characterized in that, Includes the following steps: Multi-stage surface pretreatment is performed, which includes transporting the polyethylene terephthalate film, polyethylene film and polypropylene film to be laminated to independent dielectric barrier discharge plasma treatment stations, and performing surface modification treatment under preset gas atmosphere and power density to improve the surface energy and chemical activity of each film layer. The preparation and application of a functional nanocomposite adhesive layer involves uniformly dispersing a polar copolymer matrix, a non-polar grafted polymer, and near-infrared absorbing nanoparticles in a solvent to form a functional nanocomposite adhesive. The application involves coating the adhesive onto the bonding surfaces of a pretreated polyethylene terephthalate film and a polypropylene film respectively using a microgravure coating method, and drying the coating to form an adhesive layer with controlled thickness. The primary lamination and pre-pressing process involves stacking a polyethylene terephthalate film coated with a first adhesive layer, an uncoated polyethylene film, and a polypropylene film coated with a second adhesive layer in a predetermined order, and then feeding them into a pre-pressing roller group for degassing and positioning pressing to form a multi-layer composite structure to be bonded with tightly bonded interlayers. Selective interfacial radiation fusion bonding is implemented. Selective interfacial radiation fusion bonding involves transporting the multilayer composite structure to be bonded to a high-intensity near-infrared radiation heating zone. By utilizing the penetrability of the polymer film layer to near-infrared radiation of a specific wavelength band, and the selective absorption and photothermal conversion effect of near-infrared absorbing nanoparticles in the bonding layer to that wavelength band of radiation, the two bonding interfaces are instantaneously heated to a molten state, which promotes the diffusion and entanglement of molecular chains at the interface between the bonding layer and the adjacent film layer. Forced cooling and curing are performed. Forced cooling and curing include immediately placing the multilayer composite structure into a gradient cooling system consisting of multiple temperature-controlled cooling rollers after it leaves the near-infrared radiation heating zone. Under continuous pressure, controlled cooling is carried out to complete the shaping and curing of the composite film.

2. The method according to claim 1, characterized in that, The surface modification treatment of polyethylene film and polypropylene film includes using an inert gas as a carrier gas and mixing in an oxygen-containing reactive gas to introduce oxygen-containing polar functional groups onto the surface of the polyethylene film and polypropylene film under the action of plasma.

3. The method according to claim 1, characterized in that, The surface modification treatment of the polyethylene terephthalate film includes using nitrogen as a carrier gas and mixing in an ammonia-containing reactive gas, and grafting nitrogen-containing functional groups onto the surface of the polyethylene terephthalate film under the action of plasma to enhance its chemical bonding with the adhesive layer.

4. The method according to claim 1, 2, or 3, characterized in that, The functional nanocomposite adhesive comprises: ethylene-vinyl acetate copolymer as the polar copolymer matrix, wherein the polar groups in its molecular chain form interaction forces with the modified polyethylene terephthalate film and polyethylene film surface; maleic anhydride grafted polypropylene as the non-polar grafted polymer, wherein its main chain is compatible with the polypropylene film and polyethylene film, and its grafted reactive groups chemically react with the modified film surface; and lanthanum hexaboride nanoparticles as the near-infrared absorbing nanoparticles.

5. The method according to claim 1, characterized in that, The microgravure coating method includes: using a coating roller with a preset number of micro-cells engraved on its surface; the coating roller picks up the adhesive from the material trough and scrapes off the excess material with a doctor blade, and then precisely transfers the adhesive in the cells to the surface of the film layer; subsequently, the film layer coated with wet film enters a multi-segment hot air drying oven, where the solvent evaporates at a preset temperature to form a dry film adhesive layer of uniform thickness.

6. The method according to claim 1, characterized in that, The pre-pressing roller assembly consists of a pair of rollers with surfaces coated with an elastomeric material. The rollers apply a preset linear pressure to remove interlayer air trapped during the lamination process and to ensure that each film layer remains tightly bonded before entering the near-infrared radiation heating zone, thereby ensuring uniform heat transfer.

7. The method according to claim 1, characterized in that, The high-intensity near-infrared radiation heating zone is equipped with an array of halogen lamps as a radiation source, whose emission spectrum peak wavelength matches the absorption band of the near-infrared absorbing nanoparticles. The selective interface radiation melting bonding also includes a closed-loop feedback control process. This process monitors the temperature of the multilayer composite structure interface in real time through a non-contact infrared temperature measuring device, and dynamically adjusts the output power of the halogen lamps according to the deviation between the monitored temperature and the preset target temperature to accurately maintain the interface temperature.

8. The method according to claim 1, characterized in that, The gradient cooling system includes at least three pairs of hard-coated cooling rollers with circulating cooling media inside. The multilayer composite structure passes through the at least three pairs of cooling rollers in sequence. The surface temperature of the first pair of cooling rollers is set at a relatively high temperature to perform slow cooling and optimize the interface crystal structure. The surface temperature of the subsequent cooling rollers is gradually reduced to perform rapid cooling and final shaping. Each pair of cooling rollers applies continuous linear pressure to the composite film.

9. The method according to claim 1, characterized in that, The method also includes an online quality monitoring step, which is set after the gradient cooling system. This step uses an ultrasonic scanning probe or terahertz imaging technology to continuously and non-destructively inspect the composite film that has been shaped and cured, so as to identify and mark areas with internal bonding defects or bubbles in real time.

10. The method according to claim 1, characterized in that, The near-infrared absorbing nanoparticles are antimony-doped tin dioxide nanoparticles; the high-intensity near-infrared radiation heating zone is equipped with a high-power semiconductor laser diode array as a radiation source, and the radiation center wavelength emitted by the semiconductor laser diode array matches the absorption peak of the antimony-doped tin dioxide nanoparticles.

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